drm/exynos: hdmi: add null check for hdmiphy_port

The hdmiphy can be apb and hdmiphy_port can be null. So before
accessing hdmiphy_port, it should be checked.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>